Whimsicott, in my opinion, is the best Subseeder in the game for one main reason- Prankster. But, it does have some other great qualities.
It can set up Substitute first, Stun Spore/maybe GrassWhistle opponents for instant status, and can heal first with Synthesis or heal while hurting the opponent with Giga Drain. Its typing is also great, being the only Grass/Fairy, and has many other things that can be abused, like Taunt, Tailwind, Encore, and Leech Seed.
252 HP and Defense, Bold Nature
Also, if you want a really fun option, you can use a Level One Cottonee.
Cottonee (Lv. 1)
No EVs, Nature doesn't matter
@Focus Sash, Prankster, 0 HP IVs
This set is hard to pull off, but works when it does. Use Leech Seed first so your opponent gets you down to your Focus Sash and you heal. Then, alternate between Substitute and Protect to slowly wear down your opponent and heal every turn. Taunt when you think your opponent will Taunt you or is a very bulky Pokemon.